Clean-up crews looking into a diesel spill in Kawhia Harbour yesterday will focus their efforts on finding who is responsible.

Concerned locals complaining of the strong diesel smell and a large sheen across the harbour called authorities about 1pm yesterday.

At its largest point, the sheen spread 50m by 70m, but Waikato Regional Council on-scene commander Adam Munro said the areas impacted by the spill - estimated to be at least 100 litres - was "not a huge area".

"It looks like with the tide going out and the wind going in the same direction the concentrated oil would have gone out over the entrance and conditions for breaking it up are quite favourable," he said.

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"We were talking about 100 litres at this stage and oil and water always looks more than it actually is. But obviously the locals were concerned because there was a heavy smell in the air."

A Kawhia local who asked not to be named said the diesel smell was quite strong about lunch time yesterday around the wharf and she was concerned about the harbour's prized shellfish beds.

She said she felt slightly nauseous when she went to take a look.

"It wasn't as bad as Papamoa, I was there when all that crap washed up on the beach when the ship crashed and that really was terrible. This didn't make you feel sick but of course I was still worried, it wasn't normal."

Another at the Kawhia Boat Club was not too concerned and said she saw much of the sheen rushing away with the outgoing tide.

"I think it should be all right," she said.

Mr Munro said there were some residual traces on some mud pools last night but weather and favourable sea conditions and tides should break the sheen down relatively quickly.

He said absorbent booms were available to suck up concentrated fuel although he doubted these would have to be used.

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There were also no major fears for wildlife including sea birds and seals that congregate in the harbour or out to sea.

"What's happened shouldn't really have a major impact," he said.